terminal: handle callback errors in sysview instead of propagating them

terminal: handle callback errors in sysview instead of propagating them

We cannot sanely propagate error codes if we call into user-callbacks
multiple times for multiple objects. There is no way to merge those errors
or somehow propagate them.

However, we can just act similar to sd-event and print a log-message while
discarding the values. This way, we allow error-returns, but can properly
continue working on our objects.